Press pads of this type can be used in different press arrangements, e.g. in multi-level presses with high pressure or in one level short cycle pressed with low pressure. The multi-level presses are typically used for producing synthetic material laminates made from plural paper layers which are infused with duro plastic resins under high specific pressing pressures. The multilevel presses are also used to coat particle boards or MDF or HDF plates under low pressing pressures with imprinted decorative single color craft papers that are also infused with duro plastic resins. The coating of the particle boards, MDF or HDF plates with synthetic resin films, however, is typically performed in single level presses or so called short cycle presses. The name short cycle press stands for rather short press times under pressure and temperature wherein deforming the plate material is performed in a hot condition.
The high alpha pulp papers that are infused with melamine-/formaldehyde condensation resins form solid and irreversible surfaces under pressure and temperature and thus assume a surface structure and a gloss level of metal press plates used. The press pads perform an important function when coating wood material plates with duro plastic resin films since no void free surfaces can be formed otherwise. The press pads provide even pressure distribution and a constant temperature flow. During coating in the press the duro plastic resins are initially meltable with low viscosity and flow capable and then cross link into a non-dissolvable unmeltable, hard and abrasion resistant synthetic material. This type of cross linking of the synthetic material is designated as poly condensation wherein water and formaldehyde are generated. The water generated during the condensation reaction and the formaldehyde cannot be released in vapor form since the higher pressing pressure which is above the vapor pressure of the water presses the two gaseous products into the film layer and into the carrier plate, this means the wood material. Therefore a pressing pressure is required that is uniform over an entire surface of the material to be coated. This function is performed by the press pads.
The press pads have to withstand the high temperatures that are provided in the press arrangements. Furthermore the press pads have to conduct heat quickly from the heat plate to the press plate and subsequently to the surface of the material to be pressed. Furthermore the press pads have to withstand the high pressing pressure without losing their pad properties, this means reset properties or even being destroyed.
In the press arrangements typically geometric or dimensional imperfections are provided in that the heat plates and/or the press plates are respectively not oriented exactly parallel to each other and their surfaces are not exactly flat. In case of larger deviations from the ideal geometric constellation (completely parallel orientation and flatness of the surfaces of press plates and heat pads) special pads are required which have variable thickness along their surface in order to able to compensate dimensional deviations in the press.
Press pads are typically textile fabrics that are provided with different materials and which have uniform thickness over their entire surface. In practical applications different embodiments are provided which can only partially satisfy the requirements of the respective press arrangement.
A press pad according to DEB-23 19593 includes a metal fabric which is completely coated with a non-cross linked silicone resin provided as an elastomeric material, wherein curing or cross linking is performed after coating in a drying channel and the respective surfaces have to be coated separately in a complex process.
DE 90 17 587 U1 describes a press pad which includes a flexible pad fabric that is made from an aromatic polyamide yarn and mixed with other yarn materials. With respect to a total weight of the press pad the textile fabric shall include metal threads at a rate between 0% by weight and 70% by weight so that the heat conductivity can be set to a required value.
EP 0 713 762 A2 discloses a press pad for high pressure presses and low pressure presses wherein the press pad includes various materials, e.g. yarn from a aromatic polyamide with metal threads, metal threads, heat resistant filament made from rubber or a rubber mix, heat resistant filament made from silicone or silicone mixes and their mixes with and without metal threads.
The press pad according to EP 0 725 948 B1 includes warp threads and/or weft threads which include a silicone elastomeric material which can be included in the fabric for example in a form of solid threads or in a form of metal wire that is enveloped by silicone.
Furthermore a similar press pad is known from EP 1 136 248 A1, wherein the fabric includes a substantial portion of a fluor elastomeric material and/or a silicone fluor elastomeric material. Thus, it is alternatively provided that the fabric includes a substantial portion of a blend elastomeric material which is produced by cross linking a mix made from a silicone rubber and a fluor rubber or from a silicone fluor rubber. The described elastomeric materials are thus configured as a thread material and woven into a fabric which includes interlinked warp threads or weft threads that are configured as metal threads.
EP 1 300 235 B1 describes a press pad which includes a textile carrier with threads wherein at least a portion of the threads forms heat conducting threads which cause a heat transfer from an outside of the press pad to another outside of the press pad either directly or through a contact with other heat conducting threads, wherein the carrier includes a pad layer made from an elastic rubber material in the portion of the intersecting threads. The known press pad is characterized in that a thickness of the pad layer is less than a thickness of the carrier and the pad layer is embedded into the carrier so that the heat conducting threads protrude beyond the pad layer on outsides of the press pad.
It is furthermore described that the pad layer is introduced into the textile fabric by a coating process.
Furthermore EP 1 300 235 B1 discloses that the two sided protrusion of the heat conducting threads beyond the pad layer generates a direct surface contact under the pressing pressure with the heat plates and with the press plates. This substantially accelerates heat transfer. It has become apparent under the aspect of the press and resin technology that press pads of this type with uniform fabric thickness cause problems when forming the surface of the thermosetting resin. Especially when producing floor tiles from high density fiber board (HDF) with raw thickness of 800 kg/m3 to 1000 kg/m3 press pads are required which are configured specifically for the press arrangement. This means that press pads of this type if necessary have different heat transfer coefficients over their surface on the one hand side and a different cushioning effect over their surface on the other hand side.
All previously described press pads have in common that they are produced on corresponding weaving machines and in case of DE-B-23 19 593 and EP 1 300 235 are subsequently coated with a pad material that is coatable while uncured. This means that the known press pads include a pad thickness that is constant over an entire pad surface and which can only be adapted to technical properties of the press arrangement within limits. In particular an adaptation to an individual geometric situation of a respective press arrangement is hardly possible. In press arrangements with different thickness tolerances over the surface the known press pads are deformed differently so that pressing pressures imparted onto the press plate and the press material over the surface of the press pad are different. As a consequence there is premature pad wear and a low quality surface at the coated pressing material.
It is another disadvantage of the known press pads that they are attached in the press arrangement, this means in particular at the heating plates or the press plates. Typically known press pads are fixed in the press arrangements with complex clamping devices. Thus, the press pads are subjected to very high mechanical tensions. This frequently leads to premature fabric wear, in particular for the so called top pads. Press plates are exchanged quite frequently in the press arrangements due to different surfaces that are required to form the surfaces of the thermosetting resins. Thus, the pads typically remain fixated in the arrangements. However, due to the weight of the pads substantial sagging of the pads occurs especially in the so called top plate.
When the press plates are inserted into the press arrangement again quite frequently the press pads are damaged mechanically and depending on the severity of the damage they have to be replaced with new press pads. This causes a high consumption of press pads, wherein a continued use of damaged press pads causes surface voids of the coated wood particle boards.